The present invention relates to a method of analyzing the vibrations from a drilling bit in a borehole so as to obtain information useful in managing the drilling operation.
By way of background it will be helpful first to explain the nature of a typical drilling bit. A plurality of cutters are mounted on radial axes so as to grind against the bottom of the borehole as the bit is rotated by the drill string. The cutters may have integral hardened steel teeth, which are prone to wear, or inserted teeth or studs which are highly resistant to wear. Teeth and studs may break. The bearings of the wheels are subject to wear. The teeth on a wheel are so disposed that they cannot all roll on the bottom of the borehole; instead they are forced to tear aggressively against the rock. Thus the cutters may be cones with a plurality of circumferential rows of teeth whose pitch diameters are not proportional to radial distance from the longitudinal axis of the bit. The most common bit is a tri-cone bit.
As the teeth bite against the rock one after another, they generate noise with frequency components determined by the rates at which teeth successiovely encounter the rock. It has already been appreciated that lithological information is given by the vibrational noise. At a very simple level, the harder the rock, the louder the noise. It is proposed in U.S. Pat. No. 3,626,482 (a development of U.S. Pat. No. 3,520,375) to measure the amplitude of vibrations in a frequency band or window centered on a multiple of the speed of rotation of the bit. This multiple is intended to take account of the number of "attacking elements" which are carried by the tool. Logs based on this technology have been but are no longer used by drilling companies. The above references propose detecting the vibrational energy at the top of the string or in the vicinity of the bit, in which case amplitude is transmitted up the borehole by the well known technique of mud-pulsing.
Although it is very useful to have rock hardness information since, in general, weight on bit (WOB) should be varied in proportion to rock hardness, it has now been appreciated firstly that the prior art proceeds upon an incorrect assumption and secondly that much more information can be obtained from the vibrations.
To take one important example, information regarding tooth wear could contribute significantly to the economically efficient management of a borehole. To pull out a string and replace a bit is a time-consuming operation which should desirably be conducted only at "correct" intervals, i.e. only when strictly necessary. If, to be on the safe side, a string ispulled out prematurely to change (or check) the bit, an unnecessarily high number of down days over the drilling period will result. If the bit is used for too long, at best there will be a period of inefficient drilling (maybe with a broken tooth or teeth). At worst there may be catastrophic failure with loss of a wheel, which then has to be fished out after the string has been pulled out.